In recent years, the connection of electronic data communication equipment, such as telephone facsimile devices (commonly called FAX machines) and computers, to telephone lines for data communication between such equipment has become more and more widespread as the price of the equipment has dropped. When data communication between electronic data communication equipment is desired, the equipment that is to receive data is "called" by the equipment that is to send the data. The calling is usually accomplished by an operator located at the calling station manually dialing the telephone number of the receiving station. Alternatively, more expensive electronic data communication equipment have the capability of dialing the number of the receiving station. This capability allows more expensive electronic data communication equipment to initiate data communication during periods of time when telephone system traffic is low and, as a result, line charges are low. Obviously, expensive equipment is not suitable for widespread use, particularly by untrained or relatively unsophisticated users. As a result, manual equipment is more widely used.
In order to maximize their usefulness, certain types of electronic data communication equipment, such as FAX machines, must be available to receive data at substantially any time. In order to meet this requirement, such data communication equipment is often connected to a dedicated Telco central office line, i.e., a line whose sole purpose is to convey data to and from the piece of electronic data communication equipment. The cost of a dedicated telephone line is easily justified when the line is used frequently by a piece of electronic data communication equipment. Conversely, when the telephone line is seldomly used by the piece of electronic data communication equipment, the cost of a dedicated telephone line is not easily justified. In order to avoid the expense of a dedicated line and allow a nondedicated Telco central office line to be selectively used for audio communication as well as data communication, voice/data switches have been developed. One such switch is described in U.S. Pat. No. 4,584,434 entitled "Automatic System for Automatic Information Reception Terminal Device Utilizing Telephone Circuit" by Kazuo Hashimoto. Unfortunately, voice/data switches of the type described in this patent have a serious disadvantage. Specifically, they require that a code be sent by the calling station before the piece of electronic data communication equipment that is to receive data is connected by the voice/data switch to the Telco central office line. More specifically, the switch described in U.S. Pat. No. 4,584,434 sends a message to a calling telephone station upon receipt of a ringing signal. The message requests that the calling telephone station send a particular DTMF code if the calling station desires to be connected to an associated piece of electronic data communication equipment, such as a FAX machine. If the requested code is not received, the FAX machine is not connected to the Telco line by the switch. Since there are no industry standard codes for communicating in this manner, in order for this system to be usable either an operator must initiate communication by manually calling the receiving station and responding to the message by sending the appropriate code, or the sending piece of electronic equipment must know the particular code required by the receiving station in advance and send it after the announcement has occurred. Both approaches are undesirable because they increase data communication complexity.
This invention is directed to overcoming the foregoing disadvantage by providing a voice/data switch that requires no operator or other intervention when a calling station wants to communicate with a piece of electronic data communication equipment located at a called station.